1. Field of the Invention
The present invention relates to a process for producing a disk-shaped substrate adapted for use as substrate for compact disk, video disk, optical recording disk, magnetooptical recording disk or the like.
2. Related Background Art
As schematically shown by a cross-sectional view in FIG. 1, an optical disk is prepared by forming, on a face of a substrate 1 of a disk form or doughnut form (in this text a disk form in which a central concentric portion is removed), a recording surface 2 on which information is recorded or which is capable of recording information. In case of a medium of read-only memory (ROM) type, such as the video disk, said information recording surface 2 can be formed by forming a train of pits representing information on the surface of the substrate and depositing thereon a reflective film composed for example of aluminum. In case of a DRAW (direct-read-after-write) type medium such as a magnetooptical disk, said information recording surface 2 can be formed by forming a recording layer, capable of optical information recording and reproduction, on the substrate. In this case spiral or concentric guide grooves are often formed on the surface of the substrate. Such recording surface 2 is usually formed in an annular area of a predetermined width on the substrate.
The information recorded on an optical disk is read by irradiating the disk with a light beam from a side opposite to said recording surface, and converting the change in intensity of the light beam, which is transmitted in the substrate and reflected by the recording surface, into an electrical signal. In an optical disk it is an important target to reduce the loss in said intensity change, since a large double refraction in the light-transmitting member will reduce the intensity of light emerging from the disk, thus rendering the signal reading more difficult. Such problem of double refraction is already referred to for example in the U.S. patent application Ser. No. 804,250, column 2, line 9 to column 3, line 10, filed Dec. 4, 1985, and now abandoned, which is a continuation of the U.S. patent application Ser. No. 487,890 filed Apr. 22, 1983 and subsequently abandoned, or in the U.S. patent application Ser. No. 619,339, column 2, lines 15 to 16, filed Jun. 11, 1984 and now U.S. Pat. No. 4,737,947. Consequently in the conventional process, the substrate for optical disk has been so prepared that the double refraction is substantially zero over the entire surface when the substrate is completed.
However, the substrate prepared in this manner has been associated with a drawback, even if it is free from double refraction immediately after the preparation of showing considerable double refraction in the vicinity of the external periphery and in an area somewhat distant from said external periphery, as the result of exposure to high temperature in the formation of the reflective film thereon or in the transportation of the optical disk, or as the result of extended storage.